Process of removing liquor from wood pulp digesters



Dec. 15, 1931.

5. MQRTERUD I 1,836,862

PROCESS OF REIOVING LIQUOR FRO! WOOD PULP DIGESTERS Filed Aug. 14, I930Patented Dec. 15, 1931 EINAR MORTERUD, OF TORDEROD, NEAR MOSS, NORWAYPROCESS OF REMOVING LIQUOR FROM WOOD PULP DIGESTERS Application filedAugust 14, 1930, Serial No. 475,307, and in Norway August 17, 1929.

The present invention relates to a process for washing away the liquorfrom the wood pulp digester after the termination of the digestingprocess. In case the washing liquid has a temperature which-is lowerthan the temperature of the contents of the digester,

the addition of the new liquid will have the effect of causingcondensation, pressure re duction and accordingly evaporation of theliquor which is present in the digester. In most cases this will havethe effect of reducing the thermal efficiency of the process,

as a certain quantity of heat is utilized for unnecessary evaporationinstead of directly heating of the added liquid without evaporation.

The present invention has for its object to avoid this drawback, andthis object is attained in accordance with the invention by connectingthe upper part (the steam space of the digester) with a source ofindifferent pressure gas such as air, by means of which it is possibleto regulate the pressure-in the digester independent of heat supply andheat consumption in such a manner that the addition of liquid at a lowertemperature will not cause evaporation of the hotter liquid (liquor) inthe digester.

The supply conduit for the indifferent pressure gas may be controlled byhand or may also be controlled automatically from a pressure relayconnected with the interior of the digester and by means of which it ispossible at all times to maintain independent of the temperatureconditions a constant pressure, which varies in accordance with aprescribed sca'le.

As an example of the process in accordance with the invention referenceshould be had to the diagrammatic drawing and to the followingdescription thereof.

A sulfate'digester A, in whichthe digesting process has beenterminated,is.supposed to contain wood pulp and liquor at a temperatureof 170 C. at a pressure of approximately 8 kg./cm. The digester isprovided with a bottom sieve S.

When the valve V below the sieve is opened 9 a temperature of 170 C. Inthis manner the liquor is blown out to the vessel 'B at the quantity ofliquor which may be liber ated from the wood fibres at once is removed.

In order to Wash out the rest of the liquor it is necessary to introducewashing or leaching liquid, and for .the sake of simplicity it ispresumed that there is introduced in the digester water at a temperatureof 100 C., which is spread through sieve D in form of a shower on thesurface of the wood pulp.

This Water at a temperature of 100 C.

obviously will cause condensation of the steam in the upper space of thedigester and after a certain time interval the liquor absorbed in thepulp will begin to evaporate owing to the pressure reduction caused bythe condensation. and the temperature of said liquor will thereby bereduced owing to the heat consumed by the evaporation.

If however the steam space at the top of the digester be connected witha source of pressure air such as vessel T at a pressure of 8 kg./cm. thepressure in the digester will be maintained by means of the pressure airduring the condensation of the steam.

Accordingly the washing or leaching water introduced at a temperature of100 C. will successively be substituted for the remaining liquor havinga temperature of 170 C. and at last this remaining liquor will beremoved to vessel B at full temperature, whereas at the same time thecontents of the digester will have been washed clear of liquor and befilled with pulp and water at a temperature of C. In other words theonly heat exchange that takes place will be the inconsids5 erabletransmission of heat in the zone separating the liquor and the washingwater.

If so desiredfirst amount of washing water at 100 C. may be removed inthe same manner by means of cold water maintaining the same pressure inorder to obtain a quicker re moval of the hot water.

The washing water at 100 C. more or less mixed with liquor which isremoved in th s 5 manner may be stored in a separate tank 1n order to beutilized in further operations.

In accordance with the present process there is obtained a good thermalefliciency (no depression of temperature) and an efii- 10o ing process.

cient removal of the liquor, which again means economy with regard tochemicals.

Instead of using pure water at C. it may be advantageous to use aswashing liquid a thin solution of chemicals which has a beneficenteffect on the efliciency of the wash- When this washing liquid isafterwards removed in the manner specified by means of clean cold waterin order to be utilized for further operations no loss of chemicals willtake place.

Claim: A process for removing digesting liquor from wood pulp digestersafter termination of the digesting process, by introduction of washingor leaching liquid, characterized by that washing liquid having atemperature lower than the temperature prevailing in" the interior ofthe digester is introduced into the upper part of the digester togetherwith an indiiferent pressure gas for the purpose of maintaining in theinterior of the digester a substantially constant pressure during thewashing process.

In testimony whereof I aflix my signature.

EINAR MORTERUD.

